After the wave gear device (Patent Document 1: U.S. Pat. No. 2,906,143) was invented by the original inventor C. W. Musser, a variety of inventions have been devised by many researchers. For example, wave gear devices are disclosed in Patent Document 2 (JP-B 45-41171) and Patent Document 3 (JP-A 7-167228).
A type of wave gear device having a cup-shaped or “silk hat”-shaped flexible externally toothed gear is known as the typical wave gear device. FIG. 1 is a perspective view of a cup-shaped wave gear device, and a schematic cross-sectional structural view showing a cross-section perpendicular to the axis of the wave gear device. FIG. 2 is a section through an axis showing a condition in which the opening of the cup-shaped flexible externally toothed gear has been bent into an elliptical shape, with (a) showing the pre-deformed shape, (b) being a cross-section including the major axis of the ellipse, and (c) being a cross-section including the minor axis. A “silk hat”-shaped flexible externally toothed gear is also shown in FIG. 2 by the broken line.
The cup-shaped wave gear device 1 comprises an annular rigid internally toothed gear 2, a cup-shaped flexible externally toothed gear 3 concentrically disposed inside the internally toothed gear 2, and an elliptically contoured wave generator 4 inserted in the externally toothed gear 3, as shown in the drawings. The cup-shaped flexible externally toothed gear 3 has a cylindrical body 31, an annular diaphragm 32 contiguous with one end thereof, an annular boss 33 integrally formed in the center part of the diaphragm 32, and external teeth 35 formed on the outer circumferential surface of the opening 34 of the body 31.
The diaphragm 32A of the “silk hat”-shaped flexible externally toothed gear 3A is an annular plate spreading outwardly in a radial direction, as shown by the broken line in FIG. 2. The annular boss 33A is shaped integrally along the outer circumferential edge of the diaphragm 32A.
The wave generator 4 has an elliptical rigid cam plate 41, and a wave bearing 42 fitted on the outer circumferential surface of the cam plate 41. The wave bearing 42 is composed of an inner race 42a, an outer race 42b, and a plurality of bearing balls 42c rollably mounted between the inner and outer races. The inner and outer races 42a, 42b are flexible.
The flexible externally toothed gear 3 is bent into an elliptical shape by the wave generator 4, and the external teeth 35 of the externally toothed gear 3 positioned on the major axis 3a of the ellipse mesh with the corresponding part of internal teeth 21 of the rigid internally toothed gear 2. In a cross-section perpendicular to the axis thereof, the cup-shaped or “silk hat”-shaped flexible externally toothed gear 3 (3A) repeatedly bends so as to create a deflection substantially proportional to the distance from the diaphragm, which is measured as the distance from the diaphragm to the opening. The bending deformation of the flexible externally toothed gear is referred to as “coning.”
The flexible externally toothed gear meshes with the rigid internally toothed gear and transmits the load while being elliptically deformed by the elliptically shaped wave generator inserted inside the externally toothed gear. Therefore, in order to increase the load capacity, it is necessary to increase the bottom fatigue strength of the flexible externally toothed gear. The rim thickness of the flexible externally toothed gear is an important factor influencing the bottom fatigue strength thereof. A design is proposed in Patent Document 4 for optimizing the rim thickness of the flexible externally toothed gear in order to increase the bottom fatigue strength of the annular flexible externally toothed gear in a flat-type wave gear device.
In contrast, the external teeth of an externally toothed gear in a cup-type or “silk hat”-type wave gear device are bent and caused to mesh with the internal teeth of the rigid internally toothed gear so as to create a deflection substantially proportional to the distance from the diaphragm in the direction of the flank line of the external teeth, as discussed above. Therefore, it is necessary to optimize the effective face width of the external teeth that mesh with the internal teeth in order to increase the load capacity of wave gear devices.
In other words, the gear fatigue strength in a typical gear device where rigid gears mesh together increases as the face width of the external teeth increases. In contrast, in a cup-type or “silk hat”-type wave gear device, the external teeth of the flexible externally toothed gear, which is repeatedly subjected to bending deformation called “coning,” mesh three-dimensionally with the internal teeth of the rigid internally toothed gears. The effect of the face width on the bottom fatigue strength of the flexible externally toothed gear therefore differs in accordance with the tooth profile of the external teeth of the flexible externally toothed gear in the direction of the flank line, the axial attachment position of the wave generator to the flexible externally toothed gear, and other factors. The bottom fatigue strength does not necessarily increase as the face width increases. Not only does increasing the face width beyond a reasonable limit fail to produce a satisfactory tooth flank load distribution across the entire face width, but the rigidity of the flexible externally toothed gear also increases, and elliptical deformation becomes harder to achieve, inviting the opposite effect of a decrease in the bottom fatigue strength of the flexible externally toothed gear.
It has been proposed in Patent Document 5 (JP-U 04-128558) to design the face width of the flexible externally toothed gear in a cup-type or “silk hat”-type wave gear device so that the length, measured in the flank direction, of the part where the external teeth are formed is limited to less than 24% of the pitch circle diameter thereof It has also been proposed in Patent Document 6 (JP-A 10-159917) that the length, measured in the flank direction, of the part where the external teeth are formed be reduced so as to have dimensions within the range of 5 to 14% of the pitch circle diameter thereof in order to flatten the wave gear device, and that a thin part be formed in the body adjacent to the teeth part to avoid poor meshing with the shortened external teeth.